The present invention relates to isotopic enrichment of uranium with one of its isotopes by chemical exchanges in cascade between two phases one of which is aqueous and uranium being present with different valences in the two phases.
Chemical methods of enriching uranium are known. For instance, U.K. Patent Specification No. 1,120,208 and French Patent Specification No. 1,600,437 disclose method in which the second phase is a ion exchange resin. Isotopic exchange is between tetravalent uranium IV and hexavalent uranium UVI which is present as uranyl UO.sub.2.sup.++.
According to a more efficient method, disclosed in French Patent Specification No. 2,282,928, exchange is between UIII and UIV. It is essential that the UIII containing phase be kept out of contact with electrically conductive walls and be kept free of oxidizing impurities.
It is also well known that a sufficient degree of enrichment is obtained only if a number of sub-cascades are connected for forming an enrichment cascade. If enrichment is by the chemical route, each sub-cascade comprises several contacting units in series for exchange between uranium at a first valence (+ 3 for instance) and the other phase containing uranium at the second valence ('4 for instance), an oxidizing reflow in which 235 U-depleted uranium is oxidized from the lower valence to the higher valence, and a reducing reflow in which 235 U-enriched uranium is reduced from the higher valence to the lower valence.
Two reducing approaches have been tried, namely direct electrolysis of an aqueous acid solution of a uranium salt and chemical reduction by zinc amalgam which is later electrolytically regenerated.
On the other hand, oxidation is generally by action of an oxidant gas obtained during electrolysis on an aqueous phase containing uranium at the lower valence.
One of the steps, the reducing step, consumes large amounts of power while oxidation releases heat which cannot be recovered. French Specification No. 1,600,437, which is essentially directed to chemical exchange between an aqueous phase and a second phase consisting of a fixed bed of ion exchange resin, makes a passing reference to the possibility of circulating phases containing the element to be processed through the anode compartment of an electrolytic cell (for oxidation) and the cathode compartment (for reduction). However, no illustration of that approach is given and the only example which is described consists of UIV-UVI exchange on a fixed bed of resin, reduction of UVI into UIV being by electrolysis of UO.sub.2 Cl.sub.2 and oxidation by O.sub.2 collected in the anode compartment of the electrolyser. The inventors failed to appreciate that the use of a conventional electrolyser, even with a diaphragm, for reducing UIV to UIII would not be operative and the method cannot be considered as of general use.
In addition, reduction of the element to be processed should be complete and the prior electrolysis methods are not adapted to provide acceptable Faraday efficients under such conditions.
It is an object of the present invention to provide a chemical enrichment process which makes use of exchange between UIII and UIV and of an oxidation-reduction electrolytic step without gas generation and which makes it possible to decrease the energy consumption substantially.
It is another object of the invention to provide a uranium enrichment method by the chemical exchange route which uses reduction of uranium by zinc amalgam and a combined oxidation-reduction step for regenerating the amalgam and oxidizing uranium, again without gas generation.
According to a first aspect of the invention, there is provided a process for effecting isotopic exchange between a compound of uranium at a first valence and a compound of uranium at a second valence unreactive with the first compound to produce enrichment of the lighter uranium isotope in the compound of uranium at the higher of the first and second valences, said valences being III and IV, which comprises repeating a number of times a cycle which comprises:
contacting an aqueous acid phase loaded with the compound of uranium at the first valence with a different phase loaded with the compound of uranium at the second valence, under conditions such that there is substantially no net transfer of uranium, in either valence state, from one phase to the other; PA1 extracting uranium at said second valence from said other phase, after said contact, by said aqueous phase which has previously been depleted of its uranium contents; PA1 subjecting the uranium contents in said aqueous phase, after said extraction, to a valence change from said second valence to said first valence by a process which involves electrolysis, before any new contact with said other phase; PA1 changing the valence of the uranium contained in said aqueous phase, after said contact, from said first valence to said second valence during that same electrolysis which occurs during the change of valence from said second to the first valence; PA1 and transferring uranium at said second valence into said other phase which has previously been depleted of its uranium contents. PA1 contacting an aqueous acid phase loaded with the compound of uranium at the first valence with a different phase loaded with the compound of uranium at the second valence, under conditions such that there is substantially no net transfer of uranium, in either valence state, from one phase to the other; PA1 extracting uranium at said second valence from said other phase, after said contact, by said aqueous phase which has previously been depleted of its uranium contents; PA1 subjecting the uranium contents in said aqueous phase, after said extraction, to a valence change from said second valence to said first valence; PA1 changing the valence of the uranium contained in said aqueous phase, after said contact from said first valence to said second valence; PA1 and transferring uranium at said second valence into said other phase which has previously been depleted of its uranium contents, PA1 wherein uranium is chemically reduced from the higher of said valences to the lower of said valences by contacting it with zinc amalgam having a zinc content of from 1.1% to 1.8% and said zinc amalgam is later regenerated by circulating it as a cathode in an electrolyser, while said uranium is oxidized from the lower one of said first and second valences to the higher of said valences by circulating the aqueous phase loaded with the uranium to be oxidized through the cathode compartment of the same electrolyser.
According to a first embodiment, tetravalent uranium is reduced to trivalent uranium electrolytically. According to another embodiment, UIV in an aqueous solution is reduced by zinc contained in zinc amalgam and the amalgam is electrolytically regenerated.
According to another aspect of the invention, there is provided a process for effecting isotopic exchange between a compound of uranium at a first valence and a compound of uranium at a second valence unreactive with the first compound to produce enrichment of the lighter uranium isotope in the compound of uranium at the higher of the first and second valences, which comprises repeating a number of times a cycle which comprises:
Oxidation of uranium may be carried out by electrolysis with an anode of lead, lead amalgam, graphite (if there is no free oxygen), mercury or tantalum. Lead is generally of advantage. Reduction is then carried out in the cathode compartment of the electrolyser which is fed with amalgam and with an aqueous solution of a zinc salt. The amalgam typically contains from 1.1 per cent to 1.8 per cent of zinc by weight at the output. The aqueous solution is preferably a hydrochloric solution having 4-5N of ZnCl.sub.2 at the input and 3-4N at the output. Then, there is a sufficient amount of ZnCl.sub.2 at the output for the Faraday efficiency .eta..sub.F of the electrolysis to be substantially higher than in a process where complete reduction takes place.
The invention will be better understood from the following description of preferred embodiments given by way of examples. The description refers to the accompanying drawings.